Adjuvants are substances that enhance the ability of a vaccine to elicit strong and durable immune responses to specific antigens. They are a key factor in developing subunit vaccines, and are in urgent need and a priority for vaccine research. Despite recent progress, developing subunit vaccines is still bottlenecked by the lack of safe and effective adjuvants. Discovery of novel adjuvants has emerged as a critical frontline effort in vaccine research. Saponin immune adjuvants, especially the extracts from the Quillaja Saponaria (QS) Molina tree bark, show promising adjuvant activity and are promising leads in developing structurally and compositionally defined synthetic vaccine adjuvants. Among the only four purified and characterized components of the complex tree bark extracts, the component QS-21 is one of the most sought-after human vaccine adjuvants and has been evaluated in over 100 clinical trials of vaccines against cancers and various infectious diseases. It is also a key component of the new adjuvant systems (e.g., AS01 and AS15) being tested in over 20 current clinical trials by the pharmaceutical industry. However, naturally occurring QS adjuvants have inherent drawbacks such as chemical instability, limited supply, difficult and low-yielding purification, and dose-limiting toxicity, which are the serious hurdles to their wider clinical use. In spite of recent efforts and progress in circumventing the limitations of QS-21, there remains an imperative need of structurally defined and homogeneous QS saponin-based adjuvants with enhanced adjuvant activity, reduced toxicity, simplified formulation, and much improved chemical stability and synthetic accessibility. Chemical synthesis is currently the only viable way to access the variety of structurally defined unnatural QS saponins analogs in a sufficient amount for adjuvant screenings and potential clinical application. In this application, we will focus on the design, synthesis and immunological evaluations of new QS analogs. Specific aim 1: To synthesize and evaluate QS saponin-based adjuvants with a side chain incorporated in the west wing glucuronic acid unit. Specific aim 2: To synthesize and evaluate QS saponin-based adjuvants with a galacturonic acid unit in place of the east wing native fucose unit for easy side chain attachment. Specific aim 3: To evaluate the QS saponin-based synthetic adjuvants.